Breaking an animated CAPTCHA scheme

Abstract

CAPTCHAs have become a ubiquitous security countermeasure to protect online web-services against automated attacks. However, attackers have managed to successfully break many existing CAPTCHA schemes. Animated CAPTCHA schemes have been proposed as a method of producing CAPTCHAs that are more human usable and more secure. The addition of the time dimension is supposed to increase the robustness of animated CAPTCHAs. This paper investigates the robustness of HelloCaptcha, an animated text-based CAPTCHA scheme with a total of 84 different variations. In this paper, we show that simple techniques can be used to extract important information from the animation frames of an animated CAPTCHA. Our approach essentially reduces the animated CAPTCHA into a traditional single image CAPTCHA challenge. Furthermore, the methods presented in this paper can be generalized to break other animated CAPTCHAs. © 2012 Springer-Verlag.